The heater and defroster and vents work fine in our 2002 Merc Sable. After driving for a while, the windows fog over some. It helps if I crack a window. I am wondering if a cabin filter is plugged thereby not letting in fresh air, but causing it to recirculate humid cabin air. I had a freind that had his cabin filter changed (I think it was just in front of the windshield) and it leaked water after that as they didn't put it back together correctly. Do you think this could be my problem, and any tips on changing the filter. I do a lot of my own car work. Thanks, Dave

Gotta know.......... is this just water vapor or anti-freeze? If you are in a snow and ice situation there have been some extremes around the US anyway and you would have to melt the whole car in a garage and see if a problem persists after that. Heat should be using outside air and the source could be full of ice and water near filter at cowl near wiper blades,

T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/

It's just water vapor.
The car spends all of it's time in a heated garage (above 45 degrees) when not being used. So nothing is frozen up on it.
What about the cabin filter being plugged idea?
Thanks. Dave

I can see a cabin filter causing a moisture problem if mis installed but not likely if you haven't touched it. I don't know for each car exactly but I'd take it out and replace it anyway and take a look around there for water or anything wrong. If it was plugged the volume of air thru it would be affected so I doubt it's just plain plugged. Owner's manual or perhaps right at the cover for it may show how it's replaced. Probably no tools needed but don't know,

T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/